1. Field of the Invention
The invention relates to a multilayer circuit substrate, and typically relates to a motor drive circuit substrate using the same for use in an electric power steering device.
2. Description of the Related Art
An electric power steering device for a vehicle drives a steering assist motor so as to obtain an advantageous steering assist force with respect to a steering torque applied to a steering wheel by a driver and a vehicle speed. The steering assist motor is driven by a motor drive circuit contained in an electronic control unit. When the steering assist motor is driven, the motor drive circuit controls a high power of about 500 W to 2000 W. The electronic control unit will be referred to hereinbelow as ECU.
In the above-described process, the motor drive circuit generates heat, and in order to prevent the generated heat from causing malfunction or failure of the ECU, the motor drive circuit is mounted on a circuit substrate with good thermal conductivity. For example, as shown in FIG. 13A, the motor drive circuit is mounted on a metallic circuit substrate in which a copper conductive layer 91 and a resin insulating layer 92 are formed layer by layer on an aluminum metal substrate 93 that is a heat sink.
In the circuit substrate shown in FIG. 13A, electronic components can be mounted only on one side. More specifically, electronic components can be mounted only on the conductive layer 91. Therefore, the occupation surface area of this circuit substrate in the ECU increases. Accordingly, a method of increasing the number of layers in the circuit substrate, as shown in FIG. 13B, can be considered as one of the methods for reducing the substrate surface area. In a ceramic multilayer substrate shown in FIG. 13B, a large number of copper conductive layers 91 and ceramic insulating layers 94 are laminated, an insulating resin 95 for adhesive bonding is sandwiched between the laminate and the aluminum metal substrate 93 serving as a heat sink, and joining is conducted by a hot-press joining method. Such increase in the number of layers makes it possible to decrease the length of circuit conductors, that is, wiring length. As a result, the inductance of circuit conductors can be reduced, thereby making it possible to suppress the occurrence of switching noise.
The following related art is known as the art that is associated with the invention of the application. International Patent WO 2008/078739 discloses a configuration of a multilayer circuit substrate that has a low cost and good thermal conductivity, wherein a conductive layer is formed on the inner surface by copper plating or the like and resin-filled heat-dissipating via holes are provided. Japanese Patent Application Publication No. 2006-158152 (JP-A-2006-158152) discloses a configuration in which a single-layer substrate carrying a power element in a bare chip state and a multilayer substrate carrying other elements are attached to respective heat sinks. Japanese Patent Application Publication No. 3-204996 (JP-A-3-204996) discloses a configuration in which a bare chip component is fixed to a heat spreader. Japanese Patent Application Publication No. 2000-155164 (JP-A-2000-155164) discloses a configuration of a tapered heat sink. Japanese Patent Application Publication No. 9-83106 (JP-A-9-83106) discloses a feature of forming slits in punching locations when a printed circuit substrate is processed by punching. Japanese Patent Application Publication No. 2003-134753 (JP-A-2003-134753) discloses a configuration in which a plurality of bus bars provided correspondingly to motor phases are held with gaps therebetween by an insulating holder.
As mentioned hereinabove, thermal conductivity of a multilayer circuit substrate is worse than that of a circuit substrate with a single metal layer. Therefore, a variety of measures, such as those described in the openly published documents described hereinabove, have been taken to ensure heat dissipation. In particular, in a case where an electronic component generating a larger amount of heat, such as a power Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), is mounted on a multilayer circuit substrate, a heat sink is attached to the electronic component itself, or a heat dissipating via hole that extends vertically from the rear surface of the electronic component is formed in the multilayer circuit substrate. These conventional methods assume that the electronic components are fixedly attached by soldering or the like to a front layer surface serving as a first layer of the multilayer circuit substrate. Therefore, where the attention is focused on thermal conduction from the electronic components to a metal base located in the lowermost layer of the multilayer circuit substrate, the heat dissipation efficiency of this conduction is very difficult to improve.